A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid via a point of common coupling. Microgrids are part of the structure for so called distributed generation aiming at producing electrical power locally from many small energy sources which may be called distributed generators (DG:s) or micro sources.
In a microgrid, system stability is improved with application of energy storage for continuous real and reactive power injection that works as a stabilizer for the microgrid. The main control philosophy for such stabilizer is real and reactive power injection based on local frequency and voltage deviation, respectively. In most scenarios, a larger storage/stabilizer is economical. However, in a microgrid, depending on growth, expansion and with higher penetration of DGs, it may be required to add a new storage/stabilizer in an existing microgrid and that leads to scenarios with multiple stabilizers in the same microgrid. Moreover, a planned multiple stabilizer scenario can also be beneficial for a microgrid with critical loads and frequency dependencies. This scenario is also realistic considering the DG participation in system damping.
For improved power quality, the energy storages should act fast but they always do have limits due to rating and energy left at time when needed. The charging and discharging rate of any particular storage unit is also limited. A drawback is that a high rate of discharge (or charge) of stored energy may cause tripping leading to power imbalance and stability problems. With multiple storages in a microgrid, thus one of the key objectives would be to share the power injection among the storages appropriately.
Control of multiple energy storages in a microgrid is generally known, e.g. WO 2013/005875 discloses a coordination control system for a microgrid having a first energy storage which can ouput a state of charge value to a central control device, and a second energy storage for controlling an output based on a command from the central control device.